Interleukin 6 (IL6) induces differentiation of murine myelomonocytic leukemia (Ml) cells into mature macrophages. This process is monitored by the sequential appearance of surface markers, induction of intracellular enzymes and morphological changes as the cells progress from blast cells to mature macrophages. Differentiation is also associated with growth arrest and accumulation of the differentiating cells in the GO/GL phase of the cel cycle. Interferon beta (IFN beta) is known to be involved in the growth arrest of Ml cells by inducing 2'5'oligoadenylate synthetase (2'5'-AS). We therefore analyzed whether IL6 has the potential to trigger the full differentiation program directly or whether its effect on MI cells is mediated through IFN beta or through the activation of genes that are typically induced by IFN beta. We first tested whether IL6 could induce IFN beta mRNA. Using a reverse transcription/polymerase chain reaction, we foun that IFN beta mRNA was induced by IL6. By Northern analysis, we determined that IL6 also caused a significant increase in 2',5'-AS gene expression. IL6, however, induced the expression of two mRNA species (1.7 and 2.4 kb), whereas IFN beta mainly induced the expression of the 1.7 kb species. Enhancement of 2',5'-AS gene expression by IL6 was observed even when protein synthesis was inhibited by cycloheximide. Furthermore, IL-6 induced growth arrest of MI cells was not inhibited by anti-IFN beta antibodies. Thus induction of 215'-AS gene expression is a primary response to IL6 and not secondary to the induction of IFN beta.